The invention relates to a refrigerator comprising a housing, a cylindrical working chamber, a cylindrical displacing member, a gap which is located between the housing and the displacing member, a regenerator which is disposed inside the displacing member, and a device alternatingly supplying the working chamber with a high-pressure gas and a low-pressure gas.
Refrigerators are low-temperature cooling machines in which thermodynamic cyclic processes take place (c.f. US-PS 29 06 01, for example). A single stage refrigerator comprises chiefly a working chamber with a displacing member. The working chamber is alternatingly connected to a high-pressure and a low-pressure gas source, so that during the forced reciprocating motion of the displacing member, the thermodynamic cyclic process (Stirling process, Gifford McMahon process, etc.) takes place. In doing so, the working gas is guided in a closed circuit. The result of this is that heat is removed from a certain area of the working chamber and the displacing member. Through two-stage refrigerators of this kind and with helium as the working gas, temperatures well under 10° K. can be produced.
A fundamental component of a refrigerator is the regenerator, through which the working gas flows before and after relaxation. The regenerator is commonly disposed within the displacing member substantially of cylindrical design. The regenerator material needs to exhibit, on the one hand, good heat storing properties so that a sufficiently high exchange of heat can take place between the working gas and the regenerator. On the other hand, both the displacing member, in particular the housing of the displacing member, as well as also the cylinder housing need to exhibit poor heat conducting properties, as otherwise the heat removed from the cold side of the working chamber and the displacing member would be replaced rapidly through heat conduction.
It is known to employ as the material for the cylindrical housing, stainless-steel. Stainless-steel exhibits, at the very low temperatures concerned here, a very low heat conductivity. However, stainless-steel as a material is not an option when the refrigerator is employed in the areas of magnetic fields (for example, within nuclear spin tomographs). In such instances the cylindrical housing is made of Novetex (plastic impregnated cotton wool fibre) or materials of similar properties. Novetex is well proven, in particular, as a material for the housing of the displacing member. As regenerator materials, mesh, balls or wool of bronze (preferably for the first stage) and lead balls (preferably for the second stage) are known.
In the instance of the refrigerators of the kind affected here, it is unavoidable that a gas flow is present also in the gap between the housing and displacing member. Said gas flow has also the disadvantageous effect, in that it contributes to the heat exchange between the cold and the warm end of the displacing member. The heat ingress into the expansion chamber (cold end of the working chamber) reduces the performance of the entire refrigerator.
In order to maintain the gas flow through the gap at a lower level compared to the gas flow through the regenerator, the designers of refrigerators of the type affected here have, in the past, employed the approach of designing this gap as narrow as possible, and/or have inserted seals. Measures of this kind are involved and thus costly. This applies in particular to seals which need to fulfil their task at extremely low temperatures. These seals consist commonly of plastic materials which shrink with increasing operating time. Maintaining of close tolerances is not possible.
A refrigerator of the aforementioned type is known from U.S. Pat. No. 5,481,879. For the purpose of reducing the disadvantages involved due to the flow through the gap it is proposed to equip either the outside surface of the displacing member or the inside surface of the housing with one or several helical grooves. Through this measure it shall be achieved that the gases dwell longer within the gap so that an improved temperature equalisation between the flowing gas and the adjacent components takes place. This solution is disadvantageous in that the gap still needs to be relatively narrow in order to achieve a helical gas flow. Moreover, a rapid heat exchange between the gas and the adjacent components does not take place, since these consist of materials which—as already detailed—not only exhibit a low heat conductivity, but also exhibit a very low heat storing capacity.
It is the task of the present invention to create a refrigerator of the aforementioned kind in which the disadvantages due to the gas flows in the gap have been removed.